Add support for building a Tree-sitter grammar at a given path (#9965)

This PR extends the extension builder—and by extension, the
`zed-extension` CLI—with support for building a Tree-sitter grammar at a
given path within the grammar repository.

Some Tree-sitter grammar repos contain multiple grammars inside of them.
For instance,
[`tree-sitter-php`](29838ad107)
has subfolders for `php` and `php_only`.

The grammar entries in `extension.toml` can now have an optional `path`
field that will be interpreted relative to the root of the grammar
repository:

```toml
[grammars.php]
repository = "https://github.com/tree-sitter/tree-sitter-php"
commit = "8ab93274065cbaf529ea15c24360cfa3348ec9e4"
path = "php"
```

This was something we supported in the old extension packaging script,
but hadn't yet carried it over when we built the new extension builder.

Release Notes:

- N/A

crates/extension/src/extension_manifest.rs

@@ -92,6 +92,8 @@ pub struct GrammarManifestEntry {
     pub repository: String,
     #[serde(alias = "commit")]
     pub rev: String,
+    #[serde(default)]
+    pub path: Option<String>,
 }
 
 #[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]